PRECIPITATION PATTERNS AND STREAMFLOW RESPONSE IN THE LUQUILLO MOUNTAINS, EASTERN PUERTO RICO

Watersheds in the Luquillo Mountains of Puerto Rico have abundant rainfall and stream discharge and provide a crucial water supply for the island. However, water availability may be affected by projected changes in regional temperature and atmospheric dynamics due to global warming. To help determine the links between climate and water availability, precipitation patterns were analyzed, and stable-isotope signatures of precipitation from different seasonal weather systems were established to identify those that are most important in maintaining streamflow and groundwater recharge. Long-term isotopic measurements indicate a disproportionately large contribution of trade-wind orographic precipitation to streamflow, highlighting the importance of this climate pattern to the hydrology of the watersheds. Groundwater isotopic composition suggests a slightly higher contribution from convective precipitation, but still smaller than in total rainfall. At the Rio Icacos site (640 m), the average length of time between rain events was 15 h, and 45% of rain events were <2 mm, reflecting the frequent small rain events of the orographic rainfall weather pattern. Analysis of precipitation at 15-minute resolution from 1992-2012 showed an increasing trend in monthly rainfall amount, while Caribbean radiosonde data showed an increase in precipitable water below 500 hPa. Rain event intensity distributions for 1992-2002 compared to 2002-2012 showed fewer low-intensity events and more medium-intensity events in the latter period. Annual stream discharge over the same period varied substantially, and was greatest in years of large storms; there was no statistically significant trend over the time period. Hydrograph separation experiments yielded information on stormflow characteristics; contributing sources were determined from water isotopes and solute chemistry. Streamflow response to rain events suggested relatively small hillslope storage capacity, so that prolonged droughts may affect these headwater areas. The evidence that trade-wind orographic showers contribute a high proportion of stream baseflow indicates that the area may undergo a change in water supply if orographic precipitation dynamics in the Caribbean are affected by future climate change.